Temperature effects on the LIDT of single- and multilayer sol-gel-derived thin film coatings

The formation of thin film single layers and multilayers from sol-gel materials at room temperature inherently yields a coating with residual organic impurities and trapped solvent. Both of these features have the potential to act as initiators or secondary activators of laser damage. The laser induced damage threshold (LIDT) of single layer anti- reflective silica and single layer high index zirconia coatings have been assessed as a function of curing temperature. A coating of 200-nm thickness deposited by dip coating was exposed to elevated temperatures in an air atmosphere from 20 degree(s)C to 350 degree(s)C and cooled to room temperature and pressure (RTP) before testing. The effect of this rise in temperature on the LIDT is discussed. Ultra Violet-Visible transmission spectra of the thin films are also presented. A clear densification process was found to occur leading to both a change in thickness and refractive index as a function of temperature. The absence of this trend in the low index silica layer is believed to be a direct consequence of the different chemistry during formation as a similar solvent is used in both the high and low index layers. The implications of this with respect to the effect on multilayers are also discussed.